As is known, an antigen is a substance which is able to trigger the forming of antibodies and to respond specifically to them. Most of the time, they are proteins, polyosides, lipids or nucleic acids. There are several kinds of antigens:
cell antigens which characterize and classify the individual who carries them;
parasitic antigens which comprise the parasite or that are secreted by a parasite
bacterial antigens which are supported by the membranes of bacteria, the corpus of bacteria, or which are secreted by bacteria
virus antigens.
The introduction of an antigen into an organism which is alien to it stimulates the production of antibodies. This is an immune reaction. Antibodies belong to the class of serum globulins and they all have the properties of proteins.
Antigen and antibody molecules display several active sites. When solutions of antigens and antibodies have predetermined concentrations, their mixture produces flocculation comprised of a dense network of antigen and antibody molecules. If there is an excess of one of the molecules the reaction is not obvious.
The process can be completed by adding substance called a complement to the complex. This complement is a unit of several serum proteins other than albumen, which reacts successively in a well-established order. The complement possesses no immunological specificity, but it increases the quantity of antigen-antibody complex and it makes it visible.
Thus, the corresponding mixture of antigen and antibody solutions, in adequate proportions, produces an insoluble complex in most instances and the medium becomes opalescent. Immunological precipitation and flocculation tests are therefore based on quantifying this opalescence.
Currently, in order to sense the presence of antigens or antibodies, one of the methods uses nephelometry (a method also called turbidmetry), or the observance of the appearance of a precipitate.
In a first method, diluted amounts of antigens and antibodies are mixed, then the appearance of a cloud is observed (immunodiffusion, electrosynaeresis, precipitation test). This phenomenon only takes place after twenty-four hours or even longer, which is too long for prescribing medical doses.
In order to reduce this delay and to reduce the amounts of antigens and antibodies being used, a gel of antibodies and antigens can be used. This actually reduces the delay by half, however, this time period is still too long.
These precipitations can be observed with a laser nephelometer, or a laser device which is used to observe the turbidness of a liquid, or its particle content of a different refraction index. This technique is essentially based on the scattering light with insoluble antigen-antibody complexes which are formed in a liquid environment. The intensity of the scattered light beam is then a function of the concentration of those complexes. These nephelometers are well known, and therefore we do not have to give a detailed description of them. In summary, a laser nephelometer basically includes (for instance see Medicinal Progress Technology 6, 2 (1979) 81-90):
a luminous source which is comprised of a laser beam that is designed to transmit a coherent, monochromatic, parallel and narrow light; a photoelectric sensor which gathers light scattered under an optimal angle, 31.8 degrees for instance, that is, if need be, connected to a photomultiplier machine; and
a device that provides results, either by memory input, or by direct display on a screen, or on a printer or on a tracing table.
Even though this device is well known, it still has many disadvantages which are basically related to the measuring cell which currently is comprised of a vat, the volume of which is about one cm.sup.3. Such a volume displays in a medical environment serious disadvantages because usually, the amount of liquid which is available is at best one cm.sup.3, and furthermore, for safety or other reasons, several measurements are conducted, especially to discover optimal precipitation conditions for the formed complex. Moreover, with this known method, the beginning and end of the reaction are detected, or the emergence of the total precipitation is detected, which requires another several hours, even with precipitation accelerating agents.